I wouldn't buy the 10D. You can find used 20D's for that price and the Canon 10D is an old camera. It's an APS-C crop sensor that only supports EF lenses where as all the new APS-C crop cameras support EF and EF-S lenses. It's not a huge deal seeing as how there's only 6-7 lenses in Canon's 60+ lens line up that are EF-S, but there's other differences between the 10D and the 20D that make it worth not buying a 10D.

I'm not too sure about the 50D, but I think I'd choose the D40 (non x) over that.

However, whatever you decide on remember that perhaps one of the most important things to consider is NOT going for cheap glass ! You can get the Nikon D40 for $400 at Amazon which is also a good deal, but there are certainly other things to consider, and you should really do the research before putting money down on something that you think is a good deal just because it's cheaper.

For instance, there are certain lenses which will NOT be used to their full potential (autofocus) on specific camera bodies. The D40 is one of those bodies, which, is not going to play well with some of the newer lenses on the market, and the newer lenses are particularly good with how fast and precisely they focus compared to others.

I'm on that side of the argument which says that saving a bit more money, will be worth the longer term investment and will pay off for you. Not to say that a camera such as the D40 can't take fantastic photos, not at all. In fact, it's more the person behind the camera, than the camera its self, but at the same time... when you learn how to utilize all of the features and functionality that a camera has to offer, you start to realize what might be either beyond your current skill level to grasp, or at the opposite, you realize that you want more from your tool, than what it has to offer.

It's precisely why I saved (well, my wife had a HUGE part in this) for a D300 rather than getting a D200 or D90, and I am SO happy that I did, because the D300 is a camera which is at just the right pace for me. It's not too simple, yet not so discouraging to use because the menu system and controls and ergonomics are fairly easy to learn. And the image quality is just a bonus, when paired with nice glass.

Again though, don't buy cheap glass. You won't be able to sell it when you're ready to upgrade, and it will just be a waste sitting around when you finally do. And believe me, you WILL notice the difference between glass, whether they be from the same, or different manufacturers. Also, don't let anybody tell you that third party lens manufacturers such as Sigma and Tokina are cheaper just because they're inferior.. it's bollocks. Nikon makes fantastic glass, but they do also have some stinkers in their arsenal. My current lens line up is:

Nikkor 50mm f1.8 (better value than the f1.4 because it's just as sharp wide open)
Nikkor 70-300 f4-5.6 VR (tremendous value. Great IQ even in not the best lighting)
Sigma 17-70 DC Macro HSM (fantastic walk around lens, great for wide shots and family portraits and even sports shots when the light is good) I do consider this lens to be my compromise lens. It's the weakest of the three, even though its IQ is very good. I just feel that it's a bit slower than I'd like, because when you zoom in, you lose that f2.8 real quick. I'd rather have the Nikkor 24-70 f1.8, but I can't afford it (yet).

No matter what you do.. don't skimp on glass ! By the way, what's your budget ?